Transmission of digital information over communication channels has been extensively studied. See, for example, A. J. Viterbi and J. K. Omura, Principles of Digital Communications and Coding, McGraw-Hill, N.Y., 1979. Among the many techniques developed to improve the speed and accuracy of digital communication have been those used for compensating for the distortion present in many such channels. An important set of such techniques are those for equalization of delay distortion and intersymbol interference.
A variety of techniques have long been used in equalizing relatively constant communication channels and adapting to changes that occur in the transmission characteristics of transmission channels due to changes in environmental and other circumstances. See, for example, J. G. Proakis, Digital Communications, McGraw-Hill, N.Y., 1989; and S. U. H. Qureshi, "Adaptive Equalization," Proceedings of the IEEE, Vol. 73, pp. 1349-1387, September, 1985. An important class of such techniques use non-linear equalizers such as decision feedback equalizers with adaptation logic to track slow variations in a channel after an initial learning phase. In this mode, it is assumed that the output of a decoder is correct with high probability. Error signals based on these output signals are then used to update the coefficients of the equalizer. Decision feedback techniques are generally described, for example, in A. Duel-Hallen and C. Heegard, "Delayed Decision-Feedback Sequence Estimation," IEEE Transactions on Communications, Vol. COM-37, pp. 428-436, May, 1989.
Recently, interim (non-final) decisions in delayed-decision (Viterbi-like) decoders have been used to adjust coefficients characterizing equalizers used in data communications. Such equalizers, including so-called blind equalizers using little or no training, are described in copending U.S. patent application Ser. No. 07/542,458 filed Jun. 22, 1990 by N. Seshadri which application is assigned to the assignee of the present invention.
Many communication channels, including cellular telephone channels, exhibit rapid changes in transmission characteristics, thereby causing great difficulty in adaptively equalizing such channels. Digital cellular systems are currently implemented in some countries, and a new proposal has been made for standards for a new digital cellular communication system in the United States. See, "Cellular System," Report IS-54, by the Electronic Industries Association (EIA), December, 1989. In the sequel, this proposal will be referred to as the "IS-54 standard" and systems of the type described therein as the "IS-54 system."
When equalizing a mobile (cellular) radio channel such as those employing IS-54 systems, fast adaptation is required, especially at high vehicle speeds. Such channels are typically characterized by Rayleigh fading, Doppler effects and delay spread. Prior art equalization techniques have been found lacking in mitigating the effects of such rapidly changing channel conditions.
While the prior art adaptive Viterbi Algorithm uses the globally best estimates of the transmitted data to update the estimates of the channel impulse response, processing used to develop these estimates necessarily introduces considerable complexity and delay. In a rapidly changing channel environment, the channel estimates so obtained may no longer be sufficiently accurate for currently processed symbols.
The present invention avoids the limitations of the prior art and provides a technical advance in permitting rapid adaptive equalization of digital cellular communication channels and other channels, including those exhibiting such fast changing characteristics.